Vanessa Trump, 40, opened the letter addressed to the president's son Monday morning at their midtown Manhattan apartment, investigators said. She called 911 and said she was coughing and felt nauseous, police said.

The Trump Organization didn't immediately respond to a request for comment.

Detectives from the New York Police Department's intelligence division and Secret Service agents were investigating.

"The Secret Service and our law enforcement partners in New York City are investigating a suspicious package addressed to one of our protectees received today in New York, New York. This is an active investigation and we cannot comment any further," Secret Service Special Agent Jeffrey Adams said in a statement.

Vanessa Trump, a former model, and Donald Trump Jr. have five children, none of whom were home at the time of the incident.

In March 2016, police detectives and FBI agents investigated a threatening letter sent to the Manhattan apartment of Donald Trump Jr.'s brother, Eric, that also contained a white powder that turned out to be harmless. Envelopes containing white powder were also sent to Trump Tower, which served as Trump's campaign headquarters, twice in 2016.

Hoax attacks using white powder play on fears that date to 2001, when letters containing deadly anthrax were mailed to news organizations and the offices of two U.S. senators. Those letters killed five people.

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